If you don't mind you could front wire a PEQ box so it will take a large battery so that shouldn't really be a problem.
Go with mid caps. They're the most versatile mags available. You could choose to load it up with 30 rounds (who says you have to load it up to 68 or 120? you can make them low-caps if need be).
If it was me doing it again, I really liked what I did which was pick an M4. At first I wanted a more exotic gun like a G36 or P90 but I realized it was either M4 or AK which was probably the best bet.
Anyways I ended up with an M4 AEG from the classifieds and I don't regret my decision.
If you have that kind of budget to spend then I really have no idea what you should choose however maybe a VFC SCAR might be a good choice or one of the G&P guns (but you'll have to do some preventative maintenance first cause apparently the gears on G&P's are really easy to strip).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE